1. Field of the Invention
The invention relates to a moiré method for measuring aberrations, particularly the distortion of an optical imaging system, and to a measuring system suitable for carrying out the method.
2. Description of the Related Art
In many fields of technology and research, use is made of optical imaging systems on which ever higher demands are being places as regards imaging quality. An example is the photolithographic production of semiconductor components and other finely structured components in the case of which structures in the submicrometer region can be produced with the aid of high-power projection objectives. An imaging optical with impermissibly high distortion can, for example, produce defects in integrated circuits, and this reduces the product yield in the case of semiconductor fabrication, thereby raising the production costs. Such projection objectives have a complicated optical design with a multiplicity of imperfect lenses, which renders it impossible as a rule for all the optical properties to be derived from theoretical calculations.
Consequently, the optical properties of an imaging system must be reliably measured. Even more onerous demands on the accuracy of the test methods used to test optical imaging systems correspond to the high demands placed on the imaging accuracy of these systems. In particular, a highly accurate measurement of distortion is mandatory.
Numerous test methods for measuring the distortion of optical systems are based on the utilization of the moiré effect. Thus, for example, for the purpose of determining the objective distortion in an image direction of a two-dimensional image field, it is known to evaluate moiré patterns which are generated with the aid of linear gratings. Arranged in this case in the object plane of the imaging system to be tested is, for example, a so-called object grating which has a transparent carrier which bears a multiplicity of parallel, nontransparent lines which form an object pattern. Arranged in the image plane of the objective is an image grating with an image pattern resembling the object pattern. The gratings are arranged perpendicular to the optical axis of the imaging system. The object pattern and the image pattern are adapted to one another in such a way that a moiré pattern with moiré fringes is produced when the object pattern is projected onto the image pattern with the aid of the imaging system.
The generation of a moiré pattern can be achieved by arranging for the ratio of the grating constants of the image grating and object grating to correspond exactly to the prescribed reproduction ratio of the objective. If one grating is rotated about the optical axis in relation to the other grating, the result is a moiré pattern composed of bright and dark fringes which run approximately perpendicular to the grating lines of the object and image gratings. The number of the fringe pairs is equal to the number of the grating lines of one grating which are crossed by a grating line of the other grating. A moiré pattern can also be generated by arranging for the grating constants of the object grating and image grating to differ slightly, for example by a few percent, taking account of the reproduction ratio. Moiré methods of this type are described, for example, in U.S. Pat. No. 5,767,959 or U.S. Pat. No. 5,973,773, which is largely identical in content.
The distortion of the objective, which is typically small by comparison with the grating constant of the image grating, can be determined, for example, by phase shifting. In this case, various moiré patterns with gratings which are in each case displaced relative to one another perpendicular to the grating lines by a fraction of the grating period are successively recorded by a camera. A periodic intensity profile is observed at each image location during the phase shifting. The relative phase of this signal at the various measuring sites is a measure of the objective distortion perpendicular to the grating lines.
The said methods can be used to determine the distortion in a single image direction perpendicular to the optical axis, specifically in the direction perpendicular to the grating lines. If it is intended to determine distortion components in other image directions, it is customary to turn the objective axially, for example by 90°, before a further measurement. A mechanically complicated measurement setup is required for this purpose.
EP 0 418 054 describes a moiré method which, in two successive steps, permits firstly the determination of the distortion components in an image direction, and thereafter the determination of the distortion components in an image direction perpendicular thereto. For this purpose, it is provided in the case of one embodiment that the object grating bears a two-dimensional object pattern in the form of a crossed grating. The image pattern is, by contrast, a one-dimensional one with a set of parallel lines. Furthermore, there is provided in the projection objective to be measured a displaceable pupil filter with the aid of which the diffractive reflections of first order in the X direction or Y direction can optionally be passed, and the remaining reflections can optionally be cut out. In order to alternate between the measuring directions, the pupil filter is displaced, on the one hand, and the image grating is rotated by 90°, on the other hand. Thus, in the case of this system, it is possible to dispense with the rotation of the objective. However, it is necessary to intervene in the projection objective in order to install a suitable pupil filter, and the holder for the image grating needs a complicated design which must permit the image grating to rotate by 90°.